The present invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium which is not only excellent in running property and durability but also capable of minimizing occurrence of drop-outs, and has a low light transmittance.
With recent tendency toward long-time recording and miniaturization of video or audio magnetic recording and reproducing apparatuses, magnetic recording media such as magnetic tapes have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, improved frequency characteristics and a lower noise level.
In particular, video tapes have been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
With respect to short wavelength signals, reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium. This fact is described, for example, on page 312 of Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Sogo Gijutsu Center Co., Ltd. (1982), xe2x80x9c. . . the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics with low noise are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness.xe2x80x9d
With the development of a thinner magnetic recording layer, a non-magnetic base film therefor such as a base film has also been required to have a small thickness from the viewpoints of miniaturization and long-time recording performance. As a result, such magnetic recording media suffer from deterioration in stiffness both in the machine direction and in the transverse direction, thereby causing problems concerning running property and durability thereof. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, xe2x80x9cHigher recording density is a large problem assigned to the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize the size of a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a base film . . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded.xe2x80x9d
Namely, the stiffness of the magnetic recording medium has a close relationship to its restraining force for inhibiting the separation of a running tape from a guide post for regulating tape positions, or its controlling force for maintaining a constant gap between the running tape with a predetermined tension and a magnetic head for magnetic recording and reproducing so as to achieve a stable operation of the recorder. When the stiffness of the magnetic recording medium is low, the above restraining force or controlling force is weakened, resulting in occurrence of abrasion or wrinkles of the running tape, in the worse case, breaking and severe damage thereof.
Therefore, it has been strongly required to improve the running property or durability of the magnetic recording medium.
In order to improve various properties of the magnetic recording medium, it has been attempted and already put into practice to form a back coat layer comprising plate-shaped non-magnetic particles and a binder resin on the surface opposite of the non-magnetic base film to its surface on which a magnetic recording layer is provided.
However, in the magnetic recording medium having such a back coat layer, since the back coat layer of the running tape is abraded by contacting with guide members or the like within cassette halves, there tends to be caused such a defect that the frequency of drop-outs is increased due to the abrasion of the back coat layer. For this reason, it has also been strongly required to provide a magnetic recording medium which can minimize occurrence of the drop-outs by inhibiting the back coat layer of the running tape from being abraded.
Meanwhile, the end portion of a magnetic recording medium such as a magnetic tape, especially a video tape, is judged by detecting a portion of the magnetic recording medium at which the light transmittance is large, by a video deck. When the light transmittance of the whole part of the magnetic recording layer becomes large by the production of a thinner magnetic recording medium or the ultrafine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the tape end portion by a video deck. For reducing the light transmittance of the whole part of a magnetic recording layer, carbon black fine particles or the like are added to the magnetic recording layer. It is, therefore, essential to add carbon black fine particles or the like to a magnetic recording layer in the present video tapes.
However, addition of a large amount of non-magnetic particles such as carbon black fine particles impairs not only the enhancement of the magnetic recording density but also the development of a thinner recording layer. In order to reduce the magnetization depth from the surface of the magnetic tape and to produce a thinner magnetic recording layer, it is strongly demanded to reduce the amount of non-magnetic particles such as carbon black fine particles which are added to a magnetic recording layer.
Consequently, it has been strongly demanded to provide a magnetic recording medium capable of exhibiting a low light transmittance even when the amount of carbon black fine particles added to the magnetic recording layer is reduced.
Further, in order to reduce not only the above-mentioned light transmittance of the magnetic recording medium but also the surface resistivity thereof, there has been hitherto proposed a method of adding carbon black fine particles to the magnetic recording layer.
The conventional magnetic recording medium to which carbon black fine particles are added, is described in detail below.
When a magnetic recording medium has a high surface resistivity, an electrostatic charge thereon tends to be increased, so that cut chips of the magnetic recording medium and dirt or dusts are attached onto the surface of the magnetic recording medium upon production or use thereof, and as a result, the frequency of xe2x80x9cdrop-outsxe2x80x9d becomes increased.
In order to decrease the surface resistivity of the magnetic recording medium to about 108 Q/cm2, a conductive compound such as carbon black fine particles has been ordinarily added to the magnetic recording layer in an amount of about 5 to 20 parts by weight based on 100 parts of magnetic particles contained in the magnetic recording layer.
The larger the amount of the carbon black fine particles added to the magnetic recording layer, the lower the light transmittance of the magnetic recording medium and the smaller the surface resistivity value thereof. However, when the amount of carbon black fine particles added which cannot contribute to improvement in magnetic properties of the magnetic recording layer, is increased, the magnetic recording medium is deteriorated in high-density recording performance, and the magnetic recording layer thereof is inhibited from being thinned. Further, when an excess amount of carbon black fine particles are present in the magnetic recording layer, these carbon black fine particles cannot be sufficiently bonded by a binder resin and, therefore, desorbed from the magnetic recording medium, resulting in increased drop-outs.
In order to further improve the running property of the magnetic recording medium, in addition to the formation of the above back coat layer, it has been attempted to cause the surface of the magnetic recording medium to be more slidable.
Namely, the running property of magnetic recording medium is ensured by incorporating a fatty acid such as myristic acid or stearic acid (hereinafter referred to merely as xe2x80x9cmyristic acidxe2x80x9d) in an amount of usually about 0.5 to 5% by weight based on the weight of magnetic particles, into the magnetic recording layer generally formed as an upper layer of the magnetic recording medium, and then allowing the myristic acid to be gradually oozed onto the surface of the magnetic recording layer so as to cause the surface to be slidable.
When the amount of the myristic acid oozed onto the surface of the magnetic recording layer is too small, it is not possible to ensure a good running property of the magnetic recording medium. On the other hand, when a too large amount of the myristic acid is added to the magnetic recording layer so as to allow a large amount of myristic acid to be oozed onto the surface thereof, the myristic acid is preferentially adsorbed onto the surface of each magnetic particle dispersed in the magnetic recording layer, thereby inhibiting the magnetic particles from being contacted with or adsorbed into resins. As a result, it is difficult to disperse the magnetic particles in vehicle. Also, the increase in amount of the myristic acid as a non-magnetic component causes deterioration in magnetic properties of the magnetic recording medium. Further, since the myristic acid acts as a plasticizer, there arise problems such as deterioration in mechanical strength of the magnetic recording medium.
With the recent tendency toward further reduction in thickness of the magnetic recording layer, the absolute amount of myristic acid which can be added to the magnetic recording layer is decreased. In addition, since the particle size of the magnetic particles have become much finer in order to meet the requirement of high-density recording, the BET specific surface area thereof is increased, so that a large amount of myristic acid is absorbed onto the surfaces of the magnetic particles. For this reason, it is more and more difficult to properly adjust the amount of the myristic acid oozed onto the surface of the magnetic recording layer so as to ensure a good running property of the magnetic recording layer.
Accordingly, it has been strongly required to further enhance a running property of the magnetic recording medium by appropriately controlling not only the amount of myristic acid oozed onto the surface of the magnetic recording layer, but also the amount of myristic acid oozed onto the surface of the back coat layer so as to cause even the rear surface of the magnetic recording medium to be slidable.
As conventional magnetic recording media which are improved in running property or durability, there have been known (1) a magnetic recording medium having a back coat layer in which plate-shaped hematite particles and carbon black fine particles acting as a solid lubricant are dispersed in a binder resin (Japanese Patent Publication (KOKOKU) No. 7-70043(1995), Japanese Patent No. 2945696, and Japanese Patent Application Laid-Open (KOKAI) Nos. 4-228108(1992), 8-129742(1996) and 11-273053(1999)); (2) a magnetic recording medium having a back coat layer in which plate-shaped magnetite particles and carbon black fine particles are dispersed in a binder resin (Japanese Patent Application Laid-Open (KOKAI) No. 9-198650(1997)); or the like.
There is also known the magnetic recording medium having a undercoat layer containing non-magnetic particles, which is formed between a non-magnetic base film and a magnetic recording layer, wherein the non-magnetic particles are black acicular composite iron-based particles obtained by adhering carbon black onto the surfaces of acicular hematite particles or acicular iron oxide hydroxide particles (Japanese Patent Application Laid-Open (KOKAI) No. 11-242812(1999) and European Patent Application Laid-Open No. 0924690).
Thus, it has been strongly demanded to provide a magnetic recording medium which is excellent in running property and durability and capable of minimizing occurrence of drop-outs, and exhibits a low light transmittance. However, there is provided no magnetic recording media which sufficiently satisfy these requirements.
Specifically, in any of the above-mentioned conventional magnetic recording media (1) and (2), it has been attempted to impart a durability thereto by increasing an elastic modulus of the coating film by incorporating the plate-shaped particles into the back coat layer, and to improve a running property thereof by incorporating carbon black fine particles having a solid lubrication property thereinto. However, since the plate-shaped particles tend to be agglomerated together mainly due to surface-to-surface contact therebetween and, therefore, tend to be present in non-uniform dispersed condition, it becomes difficult to uniformly orient the plate-shaped particles contained in the back coat layer in both the machine and transverse directions so as to be associated with each other in point-or line-contact manner. As a result, these conventional magnetic recording media cannot be sufficiently improved in running property and durability.
In particular, when a large amount of carbon black fine particles are added together with the plate-shaped particles, uniform orientation of the plate-shaped particles in both the machine and transverse directions is inhibited by the existence of the carbon black fine particles which are difficult to uniformly disperse due to the fineness thereof, resulting in formation of a defective back coat layer having portions where the plate-shaped particles are deficient. Therefore, the obtained magnetic recording medium is not only insufficient in durability, but also suffers from curling in themselves, resulting in deteriorated running property.
Further, in the above-described conventional magnetic recording medium (1), since reddish brown plate-shaped hematite particles are as the plate-shaped particles, it becomes difficult to sufficiently reduce the light transmittance of the magnetic recording medium. The above Japanese Patent Application Laid-Open (KOKAI) No. 9-198650(1997) described this fact as xe2x80x9cIt has been proposed that non-magnetic particles are incorporated in the magnetic recording medium to enhance the stiffness thereof. Examples of the magnetic recording media in which particles other than carbon black are incorporated into a back coat layer thereof, may include . . . . However, when such non-magnetic particles are used, the obtained magnetic recording media are deteriorated in light-shielding property and conductivityxe2x80x9d. Also, as apparently recognized from the Comparative Example 5 in which the plate-shaped hematite particles are used as the non-magnetic particles, such a magnetic recording medium exhibits a large light transmittance value.
In addition, in the conventional magnetic recording medium (2), since the plate-shaped magnetite particles are used as the plate-shaped particles, the obtained magnetic recording medium can show a higher effect of reducing a light transmittance than that using the above reddish brown plate-shaped hematite particles. However, the plate-shaped magnetite particles have a magnetism and, therefore, are strongly agglomerated together due to the magnetic force, so that it is difficult to uniformly orient the particles in the machine and transverse directions of the magnetic recording medium.
As a result of the present inventors"" earnest studies, it has been found that in a magnetic recording medium comprising a non-magnetic base film; a magnetic recording layer formed on one surface of the non-magnetic base film, comprising magnetic particles and a binder resin; and a back coat layer formed on a surface opposite of the non-magnetic base film to the surface on which the magnetic recording layer, comprising plate-shaped non-magnetic composite particles and a binder resin, by using as the plate-shaped non-magnetic composite particles, plate-shaped non-magnetic composite particles having an average plate surface diameter of 0.1 to 5.0 xcexcm, an average thickness of 0.001 to 0.1 xcexcm and a plate ratio (average plate surface diameter/average thickness) of 5:1 to 100:1, comprising:
plate-shaped hematite particles as core particles;
a coating layer formed on surface of the plate-shaped hematite particles, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and
a carbon black coat formed on the coating layer comprising said organosilicon compound, in an amount of from more than 1 to 30 parts by weight based on 100 parts by weight of the plate-shaped hematite particles,
the obtained magnetic recording medium is not only excellent in running property and durability but also capable of minimizing occurrence of drop-outs, and exhibits a low light transmittance. The present invention has been attained on the basis of this finding.
It is an object of the present invention to provide a magnetic recording medium which is not only excellent in running property and durability but also capable of minimizing occurrence of drop-outs, and exhibits a low light transmittance.
To accomplish the aim, in a first aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film;
a magnetic recording layer formed on one surface of the non-magnetic base film, comprising a binder resin and magnetic particles; and
a back coat layer formed on a surface opposite of the non-magnetic base film to the surface on which the magnetic recording layer is formed, comprising a binder resin and plate-shaped non-magnetic composite particles having an average plate surface diameter of 0.1 to 5.0 xcexcm, an average thickness of 0.001 to 0.1 xcexcm and a plate ratio (average plate surface diameter/average thickness) of 5:1 to 100:1, comprising:
plate-shaped hematite particles as core particles,
a coating layer formed on surface of the plate-shaped hematite particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes, and
a carbon black coat formed on the coating layer comprising at least one organosilicon compound, in an amount of from more than 1 to 30 parts by weight based on 100 parts by weight of the plate-shaped hematite particles.
In a second aspect of the present invention, there is provided a method of forming a magnetic recording medium comprising a non-magnetic base film, a magnetic recording layer comprising a binder resin and magnetic particles, and a back coat layer formed on a surface opposite of the non-magnetic base film to the surface on which the magnetic recording layer is formed, comprising a binder resin and non-magnetic particles, comprising:
using as non-magnetic particles plate-shaped non-magnetic composite particles having an average plate surface diameter of 0.1 to 5.0 xcexcm, an average thickness of 0.001 to 0.1 xcexcm and a plate ratio (average plate surface diameter/average thickness) of 5:1 to 100:1, comprising
plate-shaped hematite particles as core particles;
a coating layer formed on surface of the plate-shaped hematite particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and
a carbon black coat formed on the coating layer comprising at least one organosilicon compound, in an amount of from more than 1 to 30 parts by weight based on 100 parts by weight of the plate-shaped hematite particles.
The present invention will be described in detail below.
First, the back coat layer of the magnetic recording medium according to the present invention is described.
The back coat layer of the magnetic recording medium according to the present invention is formed on the surface opposite of the non-magnetic base film to its surface on which the magnetic recording layer is formed, and contains plate-shaped non-magnetic particles and a binder resin.
As the non-magnetic base film, there may be exemplified those presently used for the production of ordinary magnetic recording media. Specific examples of the non-magnetic base films may include synthetic resin films such as those made of polyethylene terephthalate, polyethylene, polypropylene, polycarbonates, polyethylene naphthalate, polyamides, polyamideimides, polyimides or the like; metal foils or plates such as those made of aluminum, stainless steels or the like; or various kinds of papers. The thickness of the non-magnetic base film is varied depending upon materials thereof, and is preferably in the range of 1.0 to 300 xcexcm, more preferably 2.0 to 200 xcexcm.
The plate-shaped non-magnetic particles used in the present invention are such plate-shaped non-magnetic composite particles which comprise plate-shaped hematite particles as core particles; a coating layer formed on the surfaces of the plate-shaped hematite particles, comprising organosilicon compounds selected from organosilane compounds obtained from alkoxysilane compounds, or polysiloxanes; and a carbon black coat formed on at least a part of the coating layer.
The lower limit of the average plate surface diameter of the plate-shaped hematite particles is usually 0.09 xcexcm, preferably 0.54 xcexcm, and the upper limit thereof is usually 4.99 xcexcm, preferably 2.99 xcexcm, more preferably 1.44 xcexcm; the lower limit of the average thickness of the plate-shaped hematite particles is usually 0.001 xcexcm, preferably 0.009 xcexcm, more preferably 0.017 xcexcm and the upper limit thereof is usually 0.099 xcexcm, preferably 0.089 xcexcm, more preferably 0.074 xcexcm; and the lower limit of the plate ratio (average plate surface diameter/average thickness) of the plate-shaped hematite particles is usually 5:1, preferably 8:1, more preferably 11:1, and the upper limit thereof is usually 100:1, preferably 48:1.
When the average plate surface diameter of the plate-shaped hematite particles is more than 4.99 xcexcm, the obtained plate-shaped non-magnetic composite particles becomes coarse particles, resulting in deteriorated tinting strength. As a result, it is difficult to reduce the light transmittance of the magnetic recording medium obtained using such non-magnetic plate-shaped composite particles. When the average plate surface diameter of the plate-shaped hematite particles is less than 0.09 xcexcm, such plate-shaped hematite particles tend to be agglomerated together because of the increase of the intermolecular force due to the fine particles. As a result, it is difficult to uniformly form a coating layer comprising organosilicon compounds composed of alkoxysilanes or polysiloxanes on the surface of the plate-shaped hematite particle, and to uniformly form a carbon black coat thereon.
When the average thickness of the plate-shaped hematite particles is more than 0.099 xcexcm, the obtained plate-shaped non-magnetic composite particles becomes coarse particles, resulting in deteriorated tinting strength. As a result, it is difficult to reduce the light transmittance of the magnetic recording medium obtained using such non-magnetic composite particles. When the average thickness of the plate-shaped hematite particles is less than 0.001 xcexcm, such plate-shaped hematite particles tend to be agglomerated together because of the increase of the intermolecular force due to the fine particles. As a result, it is difficult to uniformly form a coating layer comprising organosilicon compounds composed of alkoxysilanes or polysiloxanes on the surface of the plate-shaped hematite particle, and to uniformly form a carbon black coat thereon.
When the plate ratio of the plate-shaped hematite particles is more than 100:1, such particles tend to cause surface-to-surface contact with each other and be stacked together. As a result, it is difficult to uniformly form a coating layer comprising organosilicon compounds composed of alkoxysilanes or polysiloxanes on the surface of the plate-shaped hematite particle, and to uniformly form a carbon black coat thereon.
The plate-shaped hematite particles may contain Si compounds in an amount of 0.01 to 10% by weight (calculated as Si) based on the weight of the plate-shaped hematite particles.
When the content of the Si compounds is out of the above-specified range, it is difficult to control the myristic acid absorption. With the consideration of improvement of myristic acid absorption, the content of the Si compounds is preferably in the range of 0.02 to 5% by weight.
The lower limit of the BET specific surface area of the plate-shaped hematite particles as core particles is usually 1.0 m2/g, preferably 5.0 m2/g, and the upper limit thereof is usually 150 m2/g, preferably 120 m2/g, more preferably 100 m2/g. When the BET specific surface area is less than 1.0 m2/g, the plate-shaped hematite particles as core particles may become coarse particles, so that the obtained plate-shaped non-magnetic composite particles also may become coarse particles and tend to be deteriorated in tinting strength and as a result, it may be difficult to reduce the light transmittance of the magnetic recording medium obtained. On the other hand, when the BET specific surface area thereof is more than 150 m2/g, the intermolecular force between the particles may be increased due to the fineness thereof, so that it may become difficult to uniformly form a coating layer comprising organosilicon compounds composed of alkoxysilanes or polysiloxanes on the surface of the plate-shaped hematite particle, and uniformly form the carbon black coat on the surface of the coating layer composed of the organosilicon compounds.
As to the particle diameter distribution of the plate-shaped hematite particles used as core particles, the geometrical standard deviation value thereof is preferably not more than 2.5, more preferably not more than 2.0, still more preferably not more than 1.8. When the geometrical standard deviation value thereof is more than 2.5, coarse particles may be contained therein, so that it may also become difficult to uniformly form a coating layer comprising organosilicon compounds composed of alkoxysilanes or polysiloxanes on the surface of the plate-shaped hematite particle, and uniformly form the carbon black coat on the surface of the coating layer composed of the organosilicon compounds. The lower limit of the geometrical standard deviation value is 1.01 under the consideration of an industrial productivity.
The volume resistivity value of the core particles is usually about 1xc3x97107 to 1xc3x97109 xcexa9xc2x7cm.
The myristic acid adsorption of the core particles is usually 0.6 to 1.0 mg/m2, preferably 0.6 to 0.9 mg/m2.
In the plate-shaped hematite particles used as core particles in the present invention, the surfaces of the plate-shaped hematite particles as the core particles may be preliminarily coated with at least one compound selected from the group consisting of hydroxide of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon (hereinafter referred to as xe2x80x9chydroxides and/or oxides of aluminum and/or siliconxe2x80x9d), if required. In this case, since it is possible to more effectively reduce the carbon black desorption percentage, the dispersibility of the obtained plate-shaped non-magnetic composite particles in a vehicle during the production of the back coat layer may become improved as compared to those having no hydroxides and/or oxides of aluminum and/or silicon coat, so that a magnetic recording medium having more excellent durability, can be obtained.
The amount of the hydroxides and/or oxides of aluminum and/or silicon coat is preferably 0.01 to 50% by weight (calculated as Al, SiO2 or a sum of Al and SiO2) based on the weight of the hematite particles as the core particles.
When the amount of the hydroxides and/or oxides of aluminum and/or silicon coat is less than 0.01% by weight, the improvement of the dispersibility of the obtained plate-shaped non-magnetic composite particles in a vehicle cannot be achieved. On the other hand, when the amount of the hydroxides and/or oxides of aluminum and/or silicon coat is more than 50% by weight, the obtained plate-shaped non-magnetic composite particles can exhibit a good dispersibility in a vehicle, but it is meaningless because the dispersibility cannot be further improved by using such an excess amount of the hydroxides and/or oxides of aluminum and/or silicon coat.
The plate-shaped hematite particles having the hydroxides and/or oxides of aluminum and/or silicon coat may be substantially identical in a particle size, a geometrical standard deviation of particle sizes, a BET specific surface area and a blackness (L* value), to those having no hydroxides and/or oxides of aluminum and/or silicon coat.
The coating layer formed on the surface of the core particle comprises at least one organosilicon compound selected from the group consisting of (1) organosilane compounds obtained from alkoxysilane compounds; and (2) polysiloxanes or modified polysiloxanes selected from the group consisting of (2-A) polysiloxanes modified with at least one compound selected from the group consisting of polyethers, polyesters and epoxy compounds (hereinafter referred to merely as xe2x80x9cmodified polysiloxanesxe2x80x9d), and (2-B) polysiloxanes whose molecular terminal is modified with at least one group selected from the group consisting of carboxylic acid groups, alcohol groups and a hydroxyl group (hereinafter referred to merely as xe2x80x9c1 terminal-modified polysiloxanesxe2x80x9d).
The organosilane compounds (1) can be produced from alkoxysilane compounds represented by the formula (I):
R1aSiX4-axe2x80x83xe2x80x83(I) 
wherein R1 is C6H5xe2x80x94, (CH3)2CHCH2xe2x80x94 or n-CbH2b+1xe2x80x94 (wherein b is an integer of 1 to 18); X is CH3Oxe2x80x94 or C2H5Oxe2x80x94; and a is an integer of 0 to 3.
The alkoxysilane compounds may be dried or heat-treated for producing the organosilane compounds (1), for example, at a temperature of usually 40 to 200xc2x0 C., preferably 60 to 150xc2x0 C. for usually 10 minutes to 12 hours, preferably 30 minutes to 3 hours.
Specific examples of the alkoxysilane compounds may include methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethyoxysilane, diphenyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, isobutyltrimethoxysilane, decyltrimethoxysilane or the like. Among these alkoxysilane compounds, in view of the desorption percentage and the adhering effect of carbon black, methyltriethoxysilane, phenyltriethyoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane and isobutyltrimethoxysilane are preferred, and methyltriethoxysilane and methyltrimethoxysilane are more preferred.
As the polysiloxanes (2), there may be used those compounds represented by the formula (II): 
wherein R2 is Hxe2x80x94 or CH3xe2x80x94, and d is an integer of 15 to 450.
Among these polysiloxanes, in view of the desorption percentage and the adhering effect of carbon black, polysiloxanes having methyl hydrogen siloxane units are preferred.
As the modified polysiloxanes (2-A), there may be used:
(a1) polysiloxanes modified with polyethers represented by the formula (III): 
wherein R3 is xe2x80x94(xe2x80x94CH2xe2x80x94)hxe2x80x94; R4 is xe2x80x94(xe2x80x94CH2xe2x80x94)ixe2x80x94CH3; R5 is xe2x80x94OH, xe2x80x94COOH, xe2x80x94CHxe2x95x90CH2, xe2x80x94CH(CH3)xe2x95x90CH2 or xe2x80x94(xe2x80x94CH2xe2x80x94)jxe2x80x94CH3; R6 is xe2x80x94(xe2x80x94CH2xe2x80x94)kxe2x80x94CH3; g and h are an integer of 1 to 15; i, j and k are an integer of 0 to 15; e is an integer of 1 to 50; and f is an integer of 1 to 300;
(a2) polysiloxanes modified with polyesters represented by the formula (IV): 
wherein R7, R8 and R9 are xe2x80x94(xe2x80x94CH2xe2x80x94)qxe2x80x94 and may be the same or different; R10 is xe2x80x94OH, xe2x80x94COOH, xe2x80x94CHxe2x95x90CH2, xe2x80x94CH(CH3)xe2x95x90CH2 or xe2x80x94(xe2x80x94CH2xe2x80x94)rxe2x80x94CH3; R11 is xe2x80x94(xe2x80x94CH2xe2x80x94)sxe2x80x94CH3; n and q are an integer of 1 to 15; r and s are an integer of 0 to 15; exe2x80x2 is an integer of 1 to 50; and fxe2x80x2 is an integer of 1 to 300;
(a3) polysiloxanes modified with epoxy compounds represented by the formula (V): 
wherein R12 is xe2x80x94(xe2x80x94CH2xe2x80x94)vxe2x80x94; v is an integer of 1 to 15; t is an integer of 1 to 50; and u is an integer of 1 to 300; or a mixture thereof.
Among these modified polysiloxanes (2-A), in view of the desorption percentage and the adhering effect of carbon black, the polysiloxanes modified with the polyethers represented by the formula (III), are preferred.
As the terminal-modified polysiloxanes (2-B), there may be used those represented by the formula (VI): 
wherein R13 and R14 are xe2x80x94OH, R16OH or R17COOH and may be the same or different; R15 is xe2x80x94CH3 or xe2x80x94C6H5; R16 and R17 are xe2x80x94(xe2x80x94CH2xe2x80x94)yxe2x80x94; wherein y is an integer of 1 to 15; w is an integer of 1 to 200; and x is an integer of 0 to 100.
Among these terminal-modified polysiloxanes, in view of the desorption percentage and the adhering effect of carbon black, the polysiloxanes whose terminals are modified with carboxylic acid groups are preferred.
The amount of the coating layer composed of the organosilicon compounds is usually 0.02 to 5.0% by weight, preferably 0.03 to 4.0% by weight, more preferably 0.05 to 3.0% by weight (calculated as Si) based on the weight of the hematite particles coated with the organosilicon compounds.
When amount of the coating layer composed of the organosilicon compounds is less than 0.02% by weight, it may become difficult to form the carbon black coat on the coating layer in such an amount enough to improve the volume resistivity thereof. On the other hand, even when the coating amount of the organosilicon compounds is more than 5.0% by weight, a sufficient amount of carbon black can be coated on the coating layer. However, it is meaningless because the effects cannot be further improved by using such an excess amount of the organosilicon compounds.
As the carbon black fine particles used in the present invention, there may be exemplified commercially available carbon blacks such as furnace black, channel black or the like. Specific examples of the commercially available carbon blacks usable in the present invention, may include #3050, #3150, #3250, #3750, #3950, MA100, MA7, #1000, #2400B, #30, MA77, MA8, #650, MA11, #50, #52, #45, #2200B, MA600, etc. (tradename, produced by MITSUBISHI CHEMICAL CORP.), SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, SEAST FM, etc. (tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250, Raven 860, Raven 1000, Raven 1190 ULTRA, etc. (tradename, produced by COLOMBIAN CHEMICALS COMPANY), Ketchen black EC, Ketchen black EC600JD, etc. (tradename, produced by KETCHEN BLACK INTERNATIONAL COMPANY), BLACK PEARLS-L, BLACK PEARLS 1000, BLACK PEARLS 4630, VULCAN XC72, REGAL 660, REGAL 400, etc. (tradename, produced by CABOT SPECIALTY CHEMICALS INK CO., LTD.), or the like.
Further, in the consideration of more uniform coat of carbon black to the coating layer comprising at least one organosilicon compound, the carbon black fine particles having a DBP oil absorption of not more than 180 ml/100 g is preferred. Especially, there may be exemplified #3050, #3150, #3250, MA100, MA7, #1000, #2400B, #30, MA77, MA8, #650, MA11, #50, #52, #45, #2200B, MA600 (tradename, produced by MITSUBISHI CHEMICAL CORP.), SEAST 9H, SEAST 7H, SEAST 6, SEAST 3H, SEAST 300, SEAST FM (tradename, produced by TOKAI CARBON CO., LTD.), Raven 1250, Raven 860, Raven 1000, Raven 1190 ULTRA (tradename, produced by COLOMBIAN CHEMICALS COMPANY), BLACK PEARLS-L, BLACK PEARLS 1000, BLACK PEARLS 4630, REGAL 660, REGAL 400 (tradename, produced by CABOT SPECIALTY CHEMICALS INK CO., LTD.).
The lower limit of the average particle size of the carbon black fine particles used is usually 0.002 xcexcm, preferably 0.005 xcexcm, and the upper limit thereof is usually 0.05 xcexcm, preferably 0.035 xcexcm. When the average particle size of the carbon black fine particles used is less than 0.002 xcexcm, the carbon black fine particles used are too fine to be well handled.
On the other hand, when the average particle size thereof is more than 0.05 xcexcm, since the particle size of the carbon black fine particles used is much larger, it is necessary to apply a larger mechanical shear force for forming the uniform carbon black coat on the coating layer composed of the organosilicon compounds, thereby rendering the coating process industrially disadvantageous.
The amount of the carbon black coat formed is usually 1 to 30 parts by weight, preferably 3 to 25 parts by weight based on 100 parts by weight of the plate-shaped hematite particles as the core particles.
When the amount of the carbon black coat formed is less than 1 part by weight, the volume resistivity of the obtained plate-shaped non-magnetic composite particles may not be reduced. On the other hand, when the amount of the carbon black coat formed is more than 30 parts by weight, the carbon black may tend to be desorbed from the coating layer because of too much amount of the carbon black coat formed thereonto, though the obtained plate-shaped non-magnetic composite particles can show a sufficient volume resistivity value. As a result, the desorbed carbon black may inhibit the plate-shaped non-magnetic composite particles from being homogeneously dispersed in vehicle.
The thickness of carbon black coat formed is preferably not more than 0.04 xcexcm, more preferably not more than 0.03 xcexcm. still more preferably not more than 0.02 xcexcm. The lower limit thereof is more preferably 0.0001 xcexcm.
The particle shape and particle size of the plate-shaped non-magnetic composite particles are considerably varied depending upon those of the plate-shaped hematite particles as core particles. The plate-shaped non-magnetic composite particles have a similar particle shape to that of the plate-shaped hematite particles as core particle, and a slightly larger particle size than that of the plate-shaped hematite particles as core particles.
The lower limit of the average plate surface diameter of the plate-shaped non-magnetic composite particles used in the present invention is usually 0.1 xcexcm, preferably 0.55 xcexcm and the upper limit thereof is usually 5.0 xcexcm, preferably 3.0 xcexcm, more preferably 1.45 xcexcm; lower limit of the average thickness of the plate-shaped non-magnetic composite particles used in the present invention is usually 0.001 xcexcm, preferably 0.010 xcexcm. more preferably 0.18 xcexcm and the upper limit thereof is usually 0.1 xcexcm, preferably 0.090 xcexcm, more preferably 0.075 xcexcm; and the lower limit of the plate ratio (average plate surface diameter/average thickness) of the plate-shaped non-magnetic composite particles used in the present invention, is usually 5:1, preferably 8:1, more preferably 11:1, and the upper limit thereof is usually 100:1, preferably 48:1.
When the average plate surface diameter of the plate-shaped non-magnetic composite particles is less than 0.1 xcexcm, the dispersion of the composite particles in the vehicle upon production of a coating composition for the back coat layer may be difficult because of the increase of the intermolecular force due to the fine particles. As a result, the obtained magnetic recording medium is deteriorated in durability. When the average thickness of the plate-shaped non-magnetic composite particles is more than 5.0 xcexcm, the obtained particles becomes coarse particles, resulting in deteriorated tinting strength. As a result, it is difficult to reduce the light transmittance of the magnetic recording medium produced from such plate-shaped non-magnetic composite particles.
When the average thickness of the plate-shaped non-magnetic composite particles is less than 0.001 xcexcm, the dispersion of the composite particles in the vehicle upon production of a coating composition for the back coat layer may be difficult because of the increase of the intermolecular force due to the fine particles. As a result, the obtained magnetic recording medium is deteriorated in durability.
When the plate ratio of the plate-shaped non-magnetic composite particles is more than 100:1, such particles tend to cause surface-to-surface contact with each other and be stacked together. Therefore, the dispersion of the composite particles in the vehicle upon production of a coating composition for the back coat layer may also be difficult. As a result, it is difficult to obtain a magnetic recording medium having an excellent durability.
When the plate-shaped non-magnetic composite particles are produced by using the plate-shaped hematite particles containing Si compounds as core particles, it is possible to effectively control the myristic acid absorption thereof.
As to the particle size distribution of plate surface diameters of the plate-shaped non-magnetic composite particles, the geometrical standard deviation value thereof is not more than 2.5. When the geometrical standard deviation value is more than 2.5, the plate-shaped non-magnetic composite particles is inhibited from being uniformly dispersed in the vehicle by the existence of coarse particles. Consequently, the particles are present in non-uniformly dispersed state within the black coat layer, resulting in deteriorated stiffness of the obtained coating film. With the consideration of stiffness of the coating film, the geometrical standard deviation value of the plate-shaped non-magnetic composite particles is preferably not more than 2.0, more preferably not more than 1.8. With the consideration of industrial productivity, the lower limit of the geometrical standard deviation value is 1.01.
The BET specific surface area of the plate-shaped non-magnetic composite particles used in the present invention, is usually 1 to 150 m2/g, preferably 5 to 120 m2/g, more preferably 5 to 100 m2/g. When the BET specific surface area thereof is less than 1 m2/g, the obtained plate-shaped non-magnetic composite particles may be coarse, thereby deteriorating the tinting strength, so that it may become difficult to reduce the light transmittance of the magnetic recording medium. On the other hand, when the BET specific surface area is more than 150 m2/g, the plate-shaped non-magnetic composite particles tend to be agglomerated together by the increase in intermolecular force due to the reduction in particle diameter, thereby deteriorating the dispersibility in a binder resin upon production of the coating composition for the back coat layer, so that the obtained magnetic recording media may suffer from deterioration in surface smoothness, durability and electromagnetic performance.
The volume resistivity of the plate-shaped non-magnetic composite particles used in the present invention is preferably not more than 5xc3x97105 xcexa9xc2x7cm, more preferably 1xc3x97101 xcexa9xc2x7cm to 3xc3x97105 xcexa9xc2x7cm, still more preferably 1xc3x97101 xcexa9xc2x7cm to 1xc3x97105 xcexa9xc2x7cm. When the volume resistivity is more than 1xc3x97105 xcexa9xc2x7cm, it may be difficult to reduce the surface electrical resistivity value of the obtained back coat layer to a sufficiently low level.
The plate-shaped non-magnetic composite particles used in the present invention has a myristic acid adsorption of usually 0.01 to 0.5 mg/m2, preferably 0.01 to 0.45 mg/m2, more preferably 0.01 to 0.40 mg/m2.
When the myristic acid adsorption of the plate-shaped non-magnetic composite particles is less than 0.01 mg/m2, it may be difficult to control the amount of myristic acid oozed onto the surface of the back coat layer, due to less amount of myristic acid adsorbed into the plate-shaped non-magnetic composite particles. As a result, upon repeated use of the magnetic recording medium, it may be difficult to maintain a sufficiently low friction coefficient thereof for a long period of time.
When the myristic acid adsorption of the plate-shaped non-magnetic composite particles is more than 0.5 mg/m2, the amount of myristic acid adsorbed into the plate-shaped non-magnetic composite particles becomes too large. As a result, the amount of myristic acid oozed onto the surface of the back coat layer may become comparatively small, so that it may be difficult to ensure a good running property of the obtained magnetic recording medium.
The percentage of desorption of carbon black from the plate-shaped non-magnetic composite particles used in the present invention, is preferably not more than 20%, more preferably not more than 10%. When the desorption percentage of the carbon black is more than 20%, the desorbed carbon black may tend to inhibit the plate-shaped non-magnetic composite particles from being uniformly dispersed in the binder resin upon production of the coating composition for the back coat layer, so that it may become difficult to obtain magnetic recording media which are excellent in surface smoothness, durability.
The plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is coated with the hydroxides and/or oxides of aluminum and/or silicon have substantially the same particle size, geometrical standard deviation value, BET specific surface area value, volume resistivity value and myristic acid absorption as those of the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is uncoated with the hydroxides and/or oxides of aluminum and/or silicon, and exhibit a slightly less carbon black desorption percentage than that of the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is uncoated with the hydroxides and/or oxides of aluminum and/or silicon.
As the binder resin used in the present invention, the following resins which are at present generally used for the production of a back coat layer are usable: vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane resin, styrene-butadiene copolymer, butadiene-acrylonitrile copolymer, polyvinyl butyral, cellulose derivative such as nitrocellulose, polyester resin, synthetic rubber resin such as polybutadiene, epoxy resin, polyamide resin, polyisocyanate, electron radiation curing acryl urethane resin and mixtures thereof. Each of these resin binders may contain a functional group such as xe2x80x94OH, xe2x80x94COOH, xe2x80x94SO3M, xe2x80x94OPO2M2 and xe2x80x94NH2, wherein M represents H, Na or K. With the consideration of the dispersibility of the plate-shaped non-magnetic composite particles upon the production of the coating composition, a binder resin containing a functional group xe2x80x94COOH or xe2x80x94SO3M is preferable.
The back coat layer may contain, if required, additives used for the production of ordinary magnetic recording media such as a lubricant, a polishing agent and an anti-static agent in an amount of about 0.1 to 50 parts by weight based on 100 parts by weight of the binder resin.
The back coat layer of the present invention has a thickness of preferably 0.1 to 2.0 xcexcm, more preferably 0.2 to 1.5 xcexcm. When the thickness of the back coat layer is less than 0.1 xcexcm, the back coat layer tends to be insufficient in stiffness, so that it may be difficult to obtain a magnetic recording medium exhibiting a sufficient running durability. When the thickness of the back coat layer is more than 2.0 xcexcm, the back coat layer becomes too thick, thereby failing to reduce the thickness of the obtained magnetic recording medium.
The Young""s modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of the back coat layer of the present invention is usually not less than 100, preferably not less than 105. When the Young""s modulus is less than 100, the stiffness of the back coat layer may be insufficient and as a result, it may be difficult to improve the running durability of the obtained magnetic recording medium.
The linear absorption of the non-magnetic base film provided on one surface thereof with the back coat layer, is preferably 1.8 to 4.0 xcexcmxe2x88x921, more preferably 2.0 to 4.0 xcexcmxe2x88x921, and the surface resistivity value thereof is preferably 1xc3x97103 to 5xc3x97108 xcexa9/cm2, more preferably 1xc3x97103 to 5xc3x97107 xcexa9/cm2.
The magnetic recording layer of the present invention comprises magnetic particles and a binder resin.
As the magnetic particles used in the present invention, there may be exemplified magnetic iron oxide particles such as maghemite particles, magnetite particles and berthollide compound particles which are an intermediate oxide between maghemite and magnetite; particles obtained by incorporating any one or more different kinds of elements other than Fe, such as Co, Al, Ni, P, Zn, Si, B or the like in the said magnetic iron oxide particles; magnetic iron oxide particles obtained by coating the surface of the above-mentioned magnetic iron oxide particles or those containing different kinds of elements, with cobalt, both cobalt and iron or the like (hereinafter referred to merely as xe2x80x9cmagnetic cobalt-coated iron oxide particlesxe2x80x9d); magnetic metal particles containing iron as a main component; magnetic metal particles containing iron as a main component and elements other than Fe at least one selected from the group consisting of Co, Al, Ni, P, Si, Zn, B, Nd, La, Sm and Y, including magnetic iron-based alloy particles; magnetoplumbite-type ferrite particles such as plate-like ferrite particles containing Ba, Sr or Ba-Sr; plate-like magnetoplumbite-type ferrite particles obtained by incorporating divalent metals or tetravalent metals (such as Co, Ni, Zn, Mg, Mn, Nb, Cu, Ti, Sn, Zr, Mo or the like) as a coercive force-reducing agent in the plate-like magnetoplumbite-type ferrite particles; or the like.
With the consideration of the high-density recording, magnetic metal particles containing iron as a main component, magnetic cobalt-coated iron oxide particles and magnetic iron-based alloy particles containing elements other than Fe at least one selected from the group consisting of Co, Al, Ni, P, Si, Zn, B, Nd, La, Sm, Y or the like are preferable.
Especially, the magnetic metal particles containing iron as a main component comprising (i) iron and Al; (ii) iron, Co and Al, (iii) iron, Al and at least one rare-earth metal such as Nd, La and Y, or (iv) iron, Co, Al and at least one rare-earth metal such as Nd, La and Y are more preferable from the point of the durability of the magnetic recording medium. Further, the magnetic acicular metal particles containing iron as a main component comprising iron, Al and at least one rare-earth metal such as Nd, La and Y are still more preferable.
More specifically, the magnetic acicular metal particles containing iron as a main component may be exemplified as follows.
1) Magnetic acicular metal particles comprise iron; and cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles.
2) Magnetic acicular metal particles comprise iron; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles.
3) Magnetic acicular metal particles comprise iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles.
4) Magnetic acicular metal particles comprise iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
5) Magnetic acicular metal particles comprise iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
6) Magnetic acicular metal particles comprise iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
7) Magnetic acicular metal particles comprise iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
8) Magnetic acicular metal particles comprise iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
9) Magnetic acicular metal particles comprise iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
10) Magnetic acicular metal particles comprise iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
11) Magnetic acicular metal particles comprise iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
12) Magnetic acicular metal particles comprise iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
The iron content in the above particles is the balance, and is preferably 50 to 99% by weight, more preferably 60 to 95% by weight (calculated as Fe) based on the weight of the magnetic particles containing iron as a main component.
From the consideration of the excellent durability of the magnetic recording medium, it is preferred to use as magnetic particles magnetic acicular metal particles containing iron as a main component, which contain aluminum of 0.05 to 10% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component, which are present within the particle.
It is more preferable to use as magnetic particles magnetic acicular metal particles containing iron as a main component containing Al in an amount of 0.05 to 10% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles and a rare-earth metal such as Nd, La and Y in an amount of 0.05 to 10% by weight (calculated as element) based on the weight of the magnetic acicular metal particles. Especially, magnetic acicular metal particles containing iron as a main component containing Al and Nd therein are the even more preferable.
The magnetic particles may have not only an acicular shape but also a cubic shape, a plate-like shape or the like. Meanwhile, the term xe2x80x9cacicular shapexe2x80x9d used herein should be construed as including xe2x80x9cneedle shapexe2x80x9d, xe2x80x9cspindle shapexe2x80x9d, xe2x80x9crice grain shapexe2x80x9d and the like.
The magnetic particles have an average major axial diameter (or average plate surface diameter) of usually 0.01 to 0.50 xcexcm, preferably 0.03 to 0.30 xcexcm; an average minor axial diameter (or an average thickness) of usually 0.0007 to 0.17 xcexcm, preferably 0.003 to 0.10 xcexcm; and a geometrical standard deviation of major axial diameters of usually not more than 2.5, preferably 1.01 to 2.3.
When the magnetic particles have an acicular shape, the aspect ratio thereof is usually not less than 3:1, preferably not less than 5:1. In the consideration of good dispersibility of the particles in vehicle upon the production of a magnetic coating composition, the upper limit of the aspect ratio is usually 15:1, preferably 10:1.
When the magnetic particles have a plate shape, the plate ratio thereof is usually not less than 2:1, preferably not less than 3:1. In the consideration of good dispersibility of the particles in vehicle upon the production of a magnetic coating composition, the upper limit of the plate ratio is usually 20:1, preferably 15:1.
As to magnetic properties of the magnetic particles, in the case of acicular magnetic iron oxide particles or Co-coated acicular magnetic iron oxide particles, the coercive force value thereof is usually 19.9 to 135.3 kA/m (250 to 1,700 Oe), preferably 23.9 to 135.3 kA/m (300 to 1,700 Oe); and the saturation magnetization value thereof is usually 60 to 90 Am2/kg (60 to 90 emu/g), preferably 65 to 90 Am2/kg (65 to 90 emu/g).
In the case of acicular magnetic metal particles containing iron as a main component or acicular magnetic alloy particles containing iron as a main component, the coercive force value thereof is usually 63.7 to 278.5 kA/m (800 to 3,500 Oe), preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); and the saturation magnetization value thereof is usually 90 to 170 Am2/kg (90 to 170 emu/g), preferably 100 to 170 Am2/kg (100 to 170 emu/g).
In the case of plate-like magnetoplumbite-type ferrite particles, the coercive force value thereof is usually 39.8 to 318.3 kA/m (500 to 4,000 Oe), preferably 51.7 to 318.3 kA/m (650 to 4,000 Oe); and the saturation magnetization value thereof is usually 40 to 70 Am2/kg (40 to 70 emu/g), preferably 45 to 70 Am2/kg (45 to 70 emu/g).
As the binder resin, the same binder resin as that used for the production of the back coat layer is usable.
It is possible to add an additive such as a lubricant, a polishing agent, an antistatic agent, etc. which are generally used for the production of a magnetic recording medium, to the magnetic recording layer. The mixing ratio of the additive to the binder resin is preferably 0.1 to 50 parts by weight based on 100 parts by weight of the binder resin.
The thickness of the magnetic recording layer obtained by applying the magnetic coating composition on the surface of the non-magnetic base film and dried, is usually in the range of 0.01 to 5.0 xcexcm. If the thickness is less than 0.01 um, uniform coating may be difficult, so that unfavorable phenomenon such as unevenness on the coating surface may be observed. On the other hand, when the thickness exceeds 5.0 xcexcm, it may be difficult to obtain desired electromagnetic performance due to an influence of diamagnetism. The preferable thickness is in the range of 0.05 to 4.0 xcexcm.
The mixing ratio of the magnetic particles to the binder resin in the magnetic recording layer is usually 200 to 2,000 parts by weight, preferably 300 to 1,500 parts by weight based on 100 parts by weight of the binder resin.
When the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is uncoated with the hydroxides and/or oxides of aluminum and/or silicon are used as plate-shaped non-magnetic particles for the back coat layer, the obtained magnetic recording medium of the present invention exhibits a coercive force value of usually 19.9 to 318.3 kA/m (250 to 4,000 Oe), preferably 23.9 to 318.3 kA/m (300 to 4,000 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 130 to 300%, preferably 140 to 300%; a surface roughness Ra of coating film of usually not more than 12.0 nm, preferably 2.0 to 11.0 nm, more preferably 2.0 to 10.0 nm; and a linear absorption of coating film of usually 1.20 to 5.00 xcexcmxe2x88x921, preferably 1.30 to 5.00 xcexcmxe2x88x921. As to the durability of the magnetic recording medium, the running durability time thereof is usually not less than 23 minutes, preferably not less than 25 minutes; and the head contamination thereof is usually rank A or B, preferably rank A. Further, in the case of the above magnetic recording medium, the drop-out performance thereof is usually not more than 20/msec, preferably not more than 16/msec; the winding non-uniformity (winding disturbance) thereof is usually rank A or B, preferably rank A; and the curling thereof is usually rank A or B, preferably rank A.
When the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is surface-coated with the hydroxides and/or oxides of aluminum and/or silicon are used as plate-shaped non-magnetic particles for the back coat layer, the obtained magnetic recording medium of the present invention exhibits a coercive force value of usually 19.9 to 318.3 kA/m (250 to 4,000 Oe), preferably 23.9 to 318.3 kA/m (300 to 4,000 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 130 to 300%, preferably 140 to 300%; a surface roughness Ra of coating film of usually not more than 12.0 nm, preferably 2.0 to 11.0 nm, more preferably 2.0 to 10.0 nm; and a linear absorption of coating film of usually 1.20 to 5.00 xcexcmxe2x88x921, preferably 1.30 to 5.00 xcexcmxe2x88x921. As to the durability of the magnetic recording medium, the running durability time thereof is usually not less than 24 minutes, preferably not less than 26 minutes; and the head contamination thereof is usually rank A or B, preferably rank A. Further, in the case of the above magnetic recording medium, the drop-out performance thereof is usually not more than 17/msec, preferably not more than 13/msec; the winding non-uniformity thereof is usually rank A or B. preferably rank A; and the curling thereof is usually rank A or B, preferably rank A.
When acicular magnetic metal particles containing iron as a main component or acicular magnetic iron alloy particles containing iron as a main component are used as magnetic particles with the consideration of high-density recording or the like, and the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is uncoated with the hydroxides and/or oxides of aluminum and/or silicon are used as plate-shaped non-magnetic particles for the back coat layer, the obtained magnetic recording medium of the present invention exhibits a coercive force value of usually 63.7 to 278.5 kA/m (800 to 3,500 Oe), preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 185 to 300%, preferably 190 to 300%; a surface roughness Ra of coating film of usually not more than 9.5 nm, preferably 2.0 to 9.0 nm, more preferably 2.0 to 8.5 nm; and a linear absorption of coating film of usually 1.20 to 5.00 xcexcmxe2x88x921 preferably 1.30 to 5.00 xcexcmxe2x88x921. As to the durability of the above magnetic recording medium, the running durability time thereof is usually not less than 24 minutes, preferably not less than 26 minutes; and the head contamination thereof is usually rank A or B, preferably rank A. Further, in the case of the above magnetic recording medium, the drop-out performance thereof is usually not more than 15/msec, preferably not more than 11/msec; the winding non-uniformity thereof is usually rank A or B, preferably rank A; and the curling thereof is usually rank A or B, preferably rank A.
When the acicular magnetic metal particles containing iron as a main component or acicular magnetic iron alloy particles containing iron as a main component are used as magnetic particles, and the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is coated with the hydroxides and/or oxides of aluminum and/or silicon are used as plate-shaped non-magnetic particles for the back coat layer, the obtained magnetic recording medium of the present invention exhibits a coercive force value of usually 63.7 to 278.5 kA/m (800 to 3,500 Oe), preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 185 to 300%, preferably 190 to 300%; a surface roughness Ra of coating film of usually not more than 9.5 nm, preferably 2.0 to 9.0 nm, more preferably 2.0 to 8.5 nm; and a linear absorption of coating film of usually 1.20 to 5.00 xcexcmxe2x88x921, preferably 1.30 to 5.00 xcexcmxe2x88x921. As to the durability of the above magnetic recording medium, the running durability time thereof is usually not less than 25 minutes, preferably not less than 27 minutes; and the head contamination thereof is usually rank A or B, preferably rank A. Further, in the case of the above magnetic recording medium, the drop-out performance thereof is usually not more than 12/msec, preferably not more than 8/msec; the winding non-uniformity thereof is usually rank A or B, preferably rank A; and the curling thereof is usually rank A or B, preferably rank A .
The magnetic recording medium of the present invention, comprises a non-magnetic base film; a non-magnetic undercoat layer formed on the non-magnetic base film, comprising non-magnetic particles and a binder resin; and a magnetic recording layer formed on the non-magnetic undercoat layer.
As the non-magnetic particles used in the non-magnetic undercoat layer of the present invention, there may be exemplified non-magnetic inorganic particles ordinarily used for forming a non-magnetic undercoat layer in conventional magnetic recording media. Specific examples of the non-magnetic particles may include hematite particles, iron oxide hydroxide particles, titanium oxide particles, zinc oxide particles, tin oxide particles, tungsten oxide particles, silicon dioxide particles, xcex1-alumina particles, xcex2-alumina particles, xcex3-alumina particles, chromium oxide particles, cerium oxide particles, silicon carbide particles, titanium carbide particles, silicon nitride particles, boron nitride particles, calcium carbonate particles, barium carbonate particles, magnesium carbonate particles, strontium carbonate particles, calcium sulfate particles, barium sulfate particles, molybdenum disulfide particles, barium titanate particles or the like. These non-magnetic particles may be used singly or in the form of a mixture of any two or more thereof. Among them, the use of hematite particles, iron oxide hydroxide particles, titanium oxide particles and the like is preferred.
In the present invention, in order to improve the dispersibility of the non-magnetic particles in vehicle upon the production of non-magnetic coating composition, the non-magnetic particles may be surface-treated with hydroxides of aluminum, oxides of aluminum, hydroxides of silicon, oxides of silicon or the like to form a coat made of any of these compounds on the surfaces thereof. Further, the non-magnetic particles may contain Al, Ti, Zr, Mn, Sn, Sb or the like inside thereof, if required, in order to improve various properties of the obtained magnetic recording media such as light transmittance, surface resistivity, mechanical strength, surface smoothness, durability or the like.
In the consideration of surface smoothness of the obtained non-magnetic undercoat layer, the non-magnetic particles preferably have an acicular shape. The term xe2x80x9cacicular shapexe2x80x9d used herein should be construed as including xe2x80x9cneedle shapexe2x80x9d, xe2x80x9cspindle shapexe2x80x9d, xe2x80x9crice grain shapexe2x80x9d or the like.
When the non-magnetic particles are of a granular shape, the average particle size thereof is usually 0.01 to 0.3 xcexcm, preferably 0.015 to 0.25 xcexcm, more preferably 0.02 to 0.2 mm. When the non-magnetic particles are of an acicular shape, the average major axis diameter thereof is usually 0.01 to 0.3 xcexcm, preferably 0.015 to 0.25 xcexcm, more preferably 0.02 to 0.2 xcexcm. When the non-magnetic particles are of a plate-like shape, the average plate surface diameter thereof is usually 0.01 to 0.3 xcexcm, preferably 0.015 to 0.25 xcexcm, more preferably 0.02 to 0.2 xcexcm.
Further, when the non-magnetic particles are of an acicular shape, the aspect ratio thereof is usually 2:1 to 20:1, preferably 2.5:1 to 15:1, more preferably 3:1 to 10:1. When the non-magnetic particles are of a plate-like shape, the plate ratio thereof is usually 2:1 to 50:1, preferably 2.5:1 to 20:1, more preferably 3:1 to 10:1.
The thickness of the non-magnetic undercoat layer is preferably 0.2 to 10.0 xcexcm. When the thickness of the non-magnetic undercoat layer is less than 0.2 xcexcm, it may be difficult to improve the surface roughness of the non-magnetic base film, and the stiffness of a coating film formed thereon tends to be unsatisfactory. In the consideration of reduction in total thickness of the magnetic recording medium as well as the stiffness of the coating film, the thickness of the non-magnetic undercoat layer is more preferably in the range of 0.5 to 5.0 xcexcm.
As the binder resin, the same binder resin as that used for the production of the back coat layer is usable.
The mixing ratio of the non-magnetic particles to the binder resin is usually 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight based on 100 parts by weight of the binder resin.
When the content of the non-magnetic particles is as small as less than 5 parts by weight, such a non-magnetic undercoat layer in which the non-magnetic particles are uniformly and continuously dispersed may not be obtained upon coating, resulting in insufficient surface smoothness and insufficient stiffness of the non-magnetic base film. When the content of the non-magnetic particles is more than 2,000 parts by weight, the non-magnetic particles may not be sufficiently dispersed in a non-magnetic coating composition since the amount of the non-magnetic particles is too large as compared to that of the binder resin. As a result, when such a non-magnetic coating composition is coated onto the non-magnetic base film, it may become difficult to obtain a coating film having a sufficiently smooth surface. Further, since the non-magnetic particles may not be sufficiently bonded together through the binder resin, the obtained coating film tends to become brittle.
It is possible to add an additive such as a lubricant, a polishing agent, an antistatic agent, etc. which are generally used for the production of a magnetic recording medium, to the non-magnetic undercoating layer. The mixing ratio of the additive to the binder resin is preferably 0.1 to 50 parts by weight based on 100 parts by weight of the binder resin.
When the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is uncoated with the hydroxides and/or oxides of aluminum and/or silicon are used as plate-shaped non-magnetic particles for the back coat layer, the obtained magnetic recording medium having the non-magnetic undercoat layer according to the present invention exhibits a coercive force value of usually 19.9 to 318.3 kA/m (250 to 4,000 Oe), preferably 23.9 to 318.3 kA/m (300 to 4,000 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 135 to 300%, preferably 145 to 300%; a surface roughness Ra of coating film of usually not more than 11.5 nm, preferably 2.0 to 10.5 nm, more preferably 2.0 to 9.5 nm; and a linear absorption of coating film of usually 1.30 to 5.00 xcexcmxe2x88x921, preferably 1.40 to 5.00 xcexcmxe2x88x921. As to the durability of the above magnetic recording medium, the running durability time thereof is usually not less than 24 minutes, preferably not less than 26 minutes; and the head contamination thereof is usually rank A or B, preferably rank A. Further, in the case of the above magnetic recording medium, the drop-out performance thereof is usually not more than 19/msec, preferably not more than 15/msec; the winding non-uniformity thereof is usually rank A or B, preferably rank A; and the curling thereof is usually rank A or B, preferably rank A.
When the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is coated with the hydroxides and/or oxides of aluminum and/or silicon are used as plate-shaped non-magnetic particles for the back coat layer, the obtained magnetic recording medium having the non-magnetic undercoat layer according to the present invention exhibits a coercive force value of usually 19.9 to 318.3 kA/m (250 to 4,000 Oe), preferably 23.9 to 318.3 kA/m (300 to 4,000 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 135 to 300%, preferably 145 to 300%; a surface roughness Ra of coating film of usually not more than 11.5 nm, preferably 2.0 to 10.5 nm, more preferably 2.0 to 9.5 nm; and a linear absorption of coating film of usually 1.30 to 5.00 xcexcmxe2x88x921, preferably 1.40 to 5.00 xcexcmxe2x88x921. As to the durability of the above magnetic recording medium, the running durability time thereof is usually not less than 25 minutes, preferably not less than 27 minutes; and the head contamination thereof is usually rank A or B, preferably rank A. Further, in the case of the above magnetic recording medium, the drop-out performance thereof is usually not more than 16/msec, preferably not more than 12/msec; the winding non-uniformity thereof is usually rank A or B, preferably rank A; and the curling thereof is usually rank A or B, preferably rank A.
When acicular magnetic metal particles containing iron as a main component or acicular magnetic iron alloy particles containing iron as a main component are used as magnetic particles with the consideration of high-density recording or the like, and the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is uncoated with the hydroxides and/or oxides of aluminum and/or silicon are used as plate-shaped non-magnetic particles for the back coat layer, the obtained magnetic recording medium having the non-magnetic undercoat layer according to the present invention exhibits a coercive force value of usually 63.7 to 278.5 kA/m (800 to 3,500 Oe), preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 190 to 300%, preferably 195 to 300%; a surface roughness Ra of coating film of usually not more than 9.0 nm, preferably 2.0 to 8.5 nm, more preferably 2.0 to 8.0 nm; and a linear absorption of coating film of usually 1.30 to 5.00 xcexcmxe2x88x921, preferably 1.40 to 5.00 xcexcmxe2x88x921. As to the durability of the above magnetic recording medium, the running durability time thereof is usually not less than 25 minutes, preferably not less than 27 minutes; and the head contamination thereof is usually rank A or B, preferably rank A. Further, in the case of the above magnetic recording medium, the drop-out performance thereof is usually not more than 14/msec, preferably not more than 10/msec; the winding non-uniformity thereof is usually rank A or B, preferably rank A; and the curling thereof is usually rank A or B, preferably rank A.
When the acicular magnetic metal particles containing iron as a main component or acicular magnetic iron alloy particles containing iron as a main component are used as magnetic particles, and the plate-shaped non-magnetic composite particles in which the plate-shaped hematite particle as a core particle is coated with the hydroxides and/or oxides of aluminum and/or silicon are used as plate-shaped non-magnetic particles for the back coat layer, the obtained magnetic recording medium having the non-magnetic undercoat layer according to the present invention exhibits a coercive force value of usually 63.7 to 278.5 kA/m (800 to 3,500 Oe), preferably 71.6 to 278.5 kA/m (900 to 3,500 Oe); a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95, preferably 0.86 to 0.95; a gloss of coating film of usually 190 to 300%, preferably 195 to 300%; a surface roughness Ra of coating film of usually not more than 9.0 nm, preferably 2.0 to 8.5 nm, more preferably 2.0 to 8.0 nm; and a linear absorption of coating film of usually 1.30 to 5.00 xcexcmxe2x88x921, preferably 1.40 to 5.00 xcexcmxe2x88x921. As to the durability of the above magnetic recording medium, the running durability time thereof is usually not less than 26 minutes, preferably not less than 28 minutes; and the head contamination thereof is usually rank A or B, preferably rank A. Further, in the case of the above magnetic recording medium, the drop-out performance thereof is usually not more than 11/msec, preferably not more than 7/msec; the winding non-uniformity thereof is usually rank A or B, preferably rank A; and the curling thereof is usually rank A or B, preferably rank A.
Next, the process for producing the plate-shaped non-magnetic composite particles used in the present invention, is described.
The coating of the plate-shaped hematite particles as core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes, may be conducted (i) by mechanically mixing and stirring the plate-shaped hematite particles as core particles together with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes; or (ii) by mechanically mixing and stirring both the components together while spraying the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes onto the plate-shaped hematite particles as core particles. In these cases, substantially whole amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes added can be applied onto the surfaces of the plate-shaped hematite particles as core particles.
In order to uniformly coat the surfaces of the plate-shaped hematite particles as core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes, it is preferred that the plate-shaped hematite particles as core particles are preliminarily diaggregated by using a pulverizer.
As apparatus (a) for mixing and stirring the core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes to form the coating layer thereof, and (b) for mixing and stirring carbon black fine particles with the particles whose surfaces are coated with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes to form the carbon black coat, there may be preferably used those apparatus capable of applying a shear force to the particles, more preferably those apparatuses capable of conducting the application of shear force, spaturate force and compressed force at the same time.
As such apparatuses, there may be exemplified wheel-type kneaders, ball-type kneaders, blade-type kneaders, roll-type kneaders or the like. Among them, wheel-type kneaders are preferred.
Specific examples of the wheel-type kneaders may include an edge runner (equal to a mix muller, a Simpson mill or a sand mill), a multi-mull, a Stotz mill, a wet pan mill, a Conner mill, a ring muller, or the like. Among them, an edge runner, a multi-mull, a Stotz mill, a wet pan mill and a ring muller are preferred, and an edge runner is more preferred.
Specific examples of the ball-type kneaders may include a vibrating mill or the like. Specific examples of the blade-type kneaders may include a Henschel mixer, a planetary mixer, a Nawter mixer or the like. Specific examples of the roll-type kneaders may include an extruder or the like.
In order to coat the surfaces of the core particles with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes as uniformly as possible, the conditions of the above mixing or stirring treatment may be appropriately controlled such that the linear load is usually 19.6 to 1960 N/cm (2 to 200 Kg/cm), preferably 98 to 1470 N/cm (10 to 150 Kg/cm), more preferably 147 to 980 N/cm (15 to 100 Kg/cm); and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
The amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes added, is preferably 0.15 to 45 parts by weight based on 100 parts by weight of the plate-shaped hematite particles as core particles. When the amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes added is less than 0.15 part by weight, it may become difficult to form the carbon black coat in such an amount enough to improve the volume resistivity value of the obtained plate-shaped non-magnetic composite particles.
On the other hand, when the amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes added is more than 45 parts by weight, a sufficient amount of the carbon black coat can be formed on the surface of the coating layer, but it is meaningless because the volume resistivity value of the composite particles cannot be further improved by using such an excess amount of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes.
Next, the carbon black fine particles are added to the plate-shaped hematite particles coated with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes, and the resultant mixture is continuously mixed and stirred to form a carbon black coat on the surfaces of the coating layer composed of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes.
In addition by conducting the above-mentioned mixing or stirring treatment (b) of the carbon black fine particles together with the plate-shaped hematite particles coated with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes, at least a part of the alkoxysilane compounds coated on the plate-shaped hematite particles may be changed to the organosilane compounds.
It is preferred that the carbon black fine particles are added little by little and slowly, especially about 5 to 60 minutes.
In order to form carbon black coat onto the coating layer composed of the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes as uniformly as possible, the conditions of the above mixing or stirring treatment can be appropriately controlled such that the linear load is usually 19.6 to 1960 N/cm (2 to 200 Kg/cm), preferably 98 to 1470 N/cm (10 to 150 Kg/cm), more preferably 147 to 980 N/cm (15 to 100 Kg/cm); and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
The amount of the carbon black fine particles added, is preferably 1 to 30 parts by weight based on 100 parts by weight of the plate-shaped hematite particles as core particles. When the amount of the carbon black fine particles added is less than 1 part by weight, it may become difficult to form the carbon black coat in such an amount enough to improve the volume resistivity value of the obtained composite particles. On the other hand, when the amount of the carbon black fine particles added is more than 30 parts by weight, since a sufficient volume resisitivity value of the resultant composite particles can be obtained, but it is meaningless to adhere too large amount of the carbon black.
At least a part of the surface of the plate-shaped hematite particles as core particles may be coated with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon, in advance of mixing and stirring with the alkoxysilane compounds, the polysiloxanes, the modified polysiloxanes or the terminal-modified polysiloxanes.
The coating of the hydroxides and/or oxides of aluminum and/or silicon may be conducted by adding an aluminum compound, a silicon compound or both the compounds to a water suspension in which the plate-shaped hematite particles are dispersed, followed by mixing and stirring, and further adjusting the pH of the suspension, if required, thereby coating the surfaces of the plate-shaped hematite particles with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon. The thus obtained particles coated with the hydroxides and/or oxides of aluminum and/or silicon are then filtered out, washed with water, dried and pulverized. Further, the plate-shaped hematite particles coated with the hydroxides and/or oxides of aluminum and/or silicon may be subjected to post-treatments such as deaeration treatment and compaction treatment.
As the aluminum compounds, there may be exemplified aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride or aluminum nitrate, alkali aluminates such as sodium aluminate, or the like.
The amount of the aluminum compound added is 0.01 to 50.00% by weight (calculated as Al) based on the weight of the plate-shaped hematite particles. When the amount of the aluminum compound added is less than 0.01% by weight, it may be difficult to sufficiently coat the surfaces of the plate-shaped hematite particles with hydroxides or oxides of aluminum, thereby failing to improve the dispersibility in a vehicle. On the other hand, when the amount of the aluminum compound added is more than 50.00% by weight, the coating effect is saturated and, therefore, it is meaningless to add such an excess amount of the aluminum compound.
As the silicon compounds, there may be exemplified water glass #3, sodium orthosilicate, sodium metasilicate or the like.
The amount of the silicon compound added is 0.01 to 50.00% by weight (calculated as SiO2) based on the weight of the plate-shaped hematite particles. When the amount of the silicon compound added is less than 0.01% by weight, it may be difficult to sufficiently coat the surfaces of the plate-shaped hematite particles with hydroxides or oxides of silicon, thereby failing to improve the dispersibility in a vehicle. On the other hand, when the amount of the silicon compound added is more than 50.00% by weight, the coating effect is saturated and, therefore, it is meaningless to add such an excess amount of the silicon compound.
In the case where both the aluminum and silicon compounds are used in combination for the coating, the total amount of the aluminum and silicon compounds added is preferably 0.01 to 50.00% by weight (calculated as a sum of Al and SiO2) based on the weight of the plate-shaped hematite particles.
Next, the process for producing the magnetic recording medium of the present invention will be described below.
The magnetic recording medium of the present invention can be produced as follows. That is, according to the ordinary method, a magnetic coating composition containing magnetic particles, a binder resin and a solvent is applied onto a non-magnetic base film so as to form a magnetic coating film thereon, and the obtained coating film is then magnetically oriented in a magnetic field. Alternatively, after applying a non-magnetic coating composition containing non-magnetic particles, a binder resin and a solvent and then drying the non-magnetic coating composition to form a non-magnetic undercoat layer thereon, the above magnetic coating composition containing magnetic particles, a binder resin and a solvent is applied onto the non-magnetic undercoat layer so as to form a magnetic coating film thereon and then the obtained coating film is magnetically oriented in a magnetic field. Then, after the obtained magnetic layer is subjected to calender treatment, a coating composition for back coat layer is applied onto the opposite surface of the non-magnetic base film and cured so as to form a back coat layer, thereby producing the magnetic recording medium.
The kneading and dispersion of the non-magnetic coating composition, the magnetic coating composition and the coating composition for black coat layer may be performed using, for example, kneaders such as twin-screw kneader, twin-screw extruder, press kneader, twin-roll mill, triple-roll mill, or dispersing apparatuses such as ball mill, sand grinder, attritor, disper, homogenizer and ultrasonic dispersion device.
The coating of the non-magnetic coating composition, the magnetic coating composition and the coating composition for the back coat layer may be conducted using gravure coater, reverse-roll coater, slit coater, die coater or the like. The thus obtained coating film may be magnetically oriented using counter magnet, solenoid magnet or the like.
As the solvents, there may be exemplified those ordinarily used for the production of conventional magnetic recording media such as methyl ethyl ketone, toluene, cyclohexanone, methyl isobutyl ketone, tetrahydrofuran or a mixture thereof.
The total amount of the solvent(s) used in the non-magnetic coating composition, the magnetic coating composition or the coating composition for the back coat layer is 65 to 1,000 parts by weight based on 100 parts by weight of the non-magnetic particles, the magnetic particles or the plate-shaped non-magnetic composite particles. When the amount of the solvent used is less than 65 parts by weight, the viscosity of the obtained non-magnetic coating composition, the magnetic coating composition or the coating composition for the back coat layer may be too high, so that it is difficult to coat such a composition. When the amount of the solvent used is more than 1,000 parts by weight, the amount of the solvent vaporized upon coating may be too large, resulting in industrial disadvantages.
The point of the present invention is that when using as plate-shaped non-magnetic particles to be incorporated into the back coat layer, the plate-shaped non-magnetic composite particles which comprise plate-shaped hematite particles as core particles, a coating layer formed on the surfaces of the plate-shaped hematite particles, comprising organosilicon compounds composed of organosilane compounds obtained from alkoxysilanes or polysiloxanes, and a carbon black coat formed on the coating layer, and which have an average plate surface diameter of 0.1 to 5.0 xcexcm, an average thickness of 0.001 to 0.1 xcexcm and a plate ratio of 5:1 to 100:1, there can be obtained a thin magnetic recording medium which is not only excellent in running property and durability, but also capable of minimizing occurrence of drop-outs, and exhibits a low light transmittance.
The reason why the magnetic recording medium of the present invention have excellent running property and durability, is considered as follows. That is, conventional plate-shaped non-magnetic particles to be incorporated into the back coat layer generally tend to be agglomerated and stacked together due to the plate shape, and tend to be present in unevenly dispersed state within the back coat layer. Whereas, since the plate-shaped non-magnetic composite particles used in the present invention comprise plate-shaped hematite particles on which carbon black is uniformly and densely adhered through the coating layer comprising organosilicon compounds composed of organosilane compounds obtained from alkoxysilanes or polysiloxanes, the individual composite particles can have irregularities on the surfaces thereof and, therefore, can be inhibited from causing surface-to-surface contact with each other. As a result, the plate-shaped non-magnetic composite particles can be present in uniformly oriented state in both length and width directions of the back coat layer, so that the obtained magnetic tape can exhibit a high stiffness in both the machine and transverse directions and can be effectively inhibited from undergoing undesired curling phenomenon.
In particular, in the case of the plate-shaped non-magnetic composite particles containing the Si compounds inside the core particles, it is possible to effectively control the myristic acid absorption into the surface thereof to the specific preferred range. Therefore, an appropriate amount of myristic acid can be oozed onto the surface of the magnetic recording medium, resulting in more stabilized running property and durability thereof.
The reason why the magnetic recording medium of the present invention can minimize the occurrence of drop-outs, is considered as follows. In general, the carbon black fine particles incorporated as a solid lubricant into the back coat layer tend to be agglomerated together due to the fineness thereof, so that it becomes difficult to uniformly disperse the carbon black fine particles in the binder resin. As a result, the carbon black fine particles tend to be desorbed from the surface of the back coat layer. The desorption of the carbon black fine particles is one reason for causing the drop-outs. On the contrary, in the case of the plate-shaped non-magnetic composite particles used in the present invention, since carbon black is strongly adhered on the surfaces of the plate-shaped hematite particles through the coating layer formed thereon which comprises organosilicon compounds composed of organosilane compounds obtained from alkoxysilanes or polysiloxanes, it is possible to reduce the amount of carbon black desorbed from the surface of the carbon back coat layer to the level as low as possible.
Further, the reason why the amount of carbon black desorbed from the surfaces of the plate-shaped non-magnetic composite particles can be reduced, is considered as follows. That is, in the case where alkoxysilanes are used, metalloxane bonds (xe2x89xa1Sixe2x80x94Oxe2x80x94M, wherein M represents a metal atom contained in plate-shaped hematite particles, such as Si, Al or Fe) are formed between metal elements such as Si, Al, Fe or the like which exist within the plate-shaped hematite particles or on the surfaces of the plate-shaped hematite particles, and alkoxy groups contained in the alkoxysilanes on which the carbon black is adhered, thereby producing a strong bond between the organosilane compounds on which the carbon black is adhered, and the surfaces of the plate-shaped hematite particles.
Further, in the case where polysiloxanes are used, it is considered that various functional groups contained in the polysiloxanes on which the carbon black is adhered, can be strongly bonded to the surfaces of the plate-shaped hematite particles.
The reason why the magnetic recording medium of the present invention can exhibit a low light transmittance, is considered as follows. That is, the carbon black fine particles which usually act as agglomerates due to the fineness thereof can be uniformly and densely adhered onto the surfaces of the plate-shaped hematite particles through the coating layer formed thereon which comprises organosilicon compounds composed of organosilane compounds obtained from alkoxysilanes or polysiloxanes, so that inherent properties and functions of the carbon black can be exhibited more effectively.
Since the plate-shaped non-magnetic composite particles exhibiting an excellent dispersibility in vehicle and a well-controlled myristic acid absorption are used as non-magnetic particles for the back coat layer, the obtained magnetic recording medium of the present invention is not only excellent in running property and durability but also capable of minimizing occurrence of drop-outs, and exhibits a low light transmittance. Therefore, the magnetic recording medium of the present invention is suitably used as thin magnetic recording media.